1. Field of the Invention
The present invention relates to an image reading apparatus and control method therefor suitably used for, e.g., a color scanner or color facsimile apparatus having an image reading unit which has three, R, G, and B (Red, Green, and Blue) light sources and reads an original image by sequentially turning on the light sources.
2. Description of the Related Art
A color scanner or color facsimile apparatus having a reading unit capable of reading a color image by switching the emission colors of light sources has conventionally been known. This apparatus uses three, red, green, and blue (to be referred to as R, G, and B) LEDs as a light source, and a line image sensor as a reading unit. In this image sensor, light of each color emitted from the LED illuminates a portion of an original including an image, and the light reflected by the original surface is incident on a sensor IC via a lens array.
The line image sensor constituted by the sensor IC has a photoelectric conversion element such as a photodiode and a capacitor for each pixel, and converts incident light into a current to accumulate the current as charges in the capacitor. The accumulated charges are sequentially converted into a voltage and output from the sensor. The voltage output is converted into digital data by an A/D converter, and the digital data is subjected to various image processes to generate a scanner or facsimile output. Color read operation in a general color scanner having light sources of three colors is as follows. That is, the red LED is turned on to read one line in the main scanning direction, thereby obtaining the red component of one line of a color image. The green LED is turned on to obtain the green component of one line. The blue LED is turned on to obtain the blue component of one line.
In the color scanner of this type, an original is conveyed in the subscanning direction during an image read for one line by the LEDs of three colors. After one line is read by the LEDs of three colors, the original has been conveyed by one line. That is, the original read operation and the convey operation in the subscanning direction are simultaneously performed. These operations are repeated a plurality of number of times to read a color image of one original page. This read control method will be called xe2x80x9cscan and readxe2x80x9d.
In this reading system, since the LED light sources of the respective colors have different light quantities, a voltage output from the image sensor may exceed a predetermined A/D convertible range of the A/D converter. An output voltage from the image sensor must be limited to fall within the A/D convertible range of the A/D converter by adjusting the duty ratio of the ON time of the LED of each color in one period of time (to be simply referred to as period) of a line sync signal.
The LEDs are turned on in synchronism with a line sync signal but turned off at different timings in units of colors (in units of LEDs). More specifically, when the light emission quantity of LED is large, the LED is turned off soon; when the light emission quantity is small, the LED is turned off late. The OFF timings automatically determined by the CPU for controlling the apparatus. While the CPU turns on these LEDs, an original is actually read. In this read control, an original is conveyed in the subscanning direction during read of the original. Since the read period is different between the respective colors, the width of the read range in the subscanning direction is different between the respective colors.
This will be shown in FIG. 15. In FIG. 15, the ON periods (high-level portions of rectangular waves) of LEDs of the respective colors are longer in the order from R, G, and B. The light-receiving unit of the image sensor has an opening with a width corresponding to one line in the subscanning direction. The image sensor converts the light quantity reflected by an original surface facing this opening into charges, and accumulates the charges. An original surface facing the sensor opening from the ON timing to the OFF timing of the light source is an actual read range. As is apparent from FIG. 15, the width of the read range in the subscanning direction is longer in the order from R, G, and B.
In reading a monochrome image by the color facsimile apparatus, i.e., in reading a facsimile transmission image, all the light sources of three colors are simultaneously turned on as a white light source to read an image. Alternatively, an image is read using only a light source of one color, e.g., green out of light sources of three colors.
However, as described above, since the width of the read range in the subscanning direction is different between the respective colors, the resolution in the subscanning direction differs between the respective colors. As a result, read data of an originally black thin horizontal line does not indicate black, resulting in poor color reproducibility in reading a halftone original.
In the prior art, when the light sources of three colors are simultaneously turned on to read a monochrome image, the power is greatly consumed. The light emission quantity of the light source increases to require a heat dissipation structure, resulting in a large outer shape of the reading unit. Since the three light sources vary in light quantity, they do not function as a complete white light source. If an original to be read is a color original, a color close to one having a large light quantity has a low read density, and the original density cannot be accurately expressed. This problem becomes more serious :in reading an original in a pseudo halftone mode.
To the contrary, when a monochrome original is read by turning on only a light source of one color (monochrome mode), if monochrome originals are frequently read, the light source of that color degrades sooner than the remaining light sources. The degradation of the light source must be finely corrected in reading a color original (color mode). This problem becomes notable when an LED is used as a light source. When a color original is read in the monochrome mode, the same color as the light source cannot be read.
The present invention has been made to solve these problems, and has as its first object to attain the same resolution in the subscanning direction for the respective colors in reading an image and improve the color reproducibility of a black thin horizontal line or halftone original even if variations in light quantities of light sources of a plurality of colors are adjusted in a reading system of reading an original image while scanning.
It is the second object of the present invention to read a high-quality image without any dropout color with small power consumption and eliminate variations in degradation of light sources when a monochrome image is read.
To achieve the above objects, an image reading apparatus according to the present invention has the following arrangement.
More specifically, an image reading apparatus which reads an original image by sequentially irradiating an original image with a plurality of emission colors and detecting light of each color from the original image by an image sensor comprises a light source capable of sequentially generating the plurality of emission colors, and control means for setting a flickering duty ratio of the light source for each emission color, when the original image is read, changing the emission color every predetermined charge accumulation period of the image sensor, and flickering the light source in accordance with the duty ratio set for each emission color.
The control means may set a flickering duty ratio of another emission color using, as a reference, a total ON time of one of the plurality of emission colors in a predetermined charge accumulation period of the image sensor, when the original image is read, change the emission color every charge accumulation period, and flicker the light source with another emission color in accordance with the duty ratio set for each emission color.
At this time, for example, the control means calculates the flickering duty ratio of the light source for each emission color, calculates a product of the calculated duty ratio and the predetermined charge accumulation period of the image sensor to calculate the total ON time for each color, and selects a product of an emission color having the largest calculated value as the reference total ON time.
In any of the above apparatus arrangements, when a reference white background is irradiated by flickering the light source with one of the plurality of emission colors, and an output value from the image sensor is smaller than a predetermined value and comes nearest to the predetermined value, the control means sets a flickering duty ratio at this time as an optimum duty ratio for the emission color, the control means repeating the duty ratio setting operation for each of the plurality of emission colors.
Alternatively, an image reading apparatus which reads an original image by sequentially irradiating the original image with a plurality of emission colors and detecting light of each color from the original image by an image sensor comprises a light source capable of sequentially generating the plurality of emission colors, and control means for setting light emission times of the plurality of emission colors in a predetermined charge accumulation period of the image sensor, when the original image is read, sequentially changing the emission colors during the charge accumulation period, and turning on the light source in accordance with the light emission time set for each emission color.
At this time, for example, when a reference white background is irradiated by turning on the light source with one of the plurality of emission colors, and an output value from the image sensor is smaller than a predetermined value and comes nearest to the predetermined value, the control means sets a light emission time at this time as an optimum light emission time for the emission color, the control means repeating the light emission time setting operation for each of the plurality of emission colors.
Further, in a monochrome mode, the control means sequentially changes the emission colors during the charge accumulation period, and forms a pseudo white light source by turning on the light source in accordance with the light emission time set for each emission color.
To achieve the above objects, an image reading apparatus according to the present invention has the following arrangement.
More specifically, an image reading apparatus which reads an original image by detecting, by an image sensor, light from the original image irradiated by a light source comprises control means for controlling a single light source for irradiating the original image to flicker a plurality of number of times every predetermined charge accumulation time of the image sensor when the original image is read.
The control means adjusts a flickering duty ratio when the light source flickers.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.